1. Field of the Invention
This invention relates to modified unsaturated polyester compositions which have improved physical properties when used as molding compositions. In particular, the modified unsaturated polyester compositions exhibit highly advantageous hydrolytic stability and resistance to chemical degradation.
2. Description of the Prior Art
Unsaturated polyesters are industrially important materials which have broad application in the manufacture of products such as automotive parts, boats, appliance parts, and furniture. Unsaturated polyester compositions have also been applied to the fabricating of pipes, tanks and fume ducts used for handling materials in waste water treatment, selected chemicals, and gas and oil.
One problem which has been encountered in the past with such compositions is that of high polymerization shrinkage of the polyester resin. Shrinkage results in poor surface smoothness and low gloss of molded parts. Polymerization shrinkage also results in poor dimensional agreement between the mold surface and the fabricated part. Another problem encountered is that crosslinked unsaturated polyester matrix is a brittle glassy phase that has low chip and crack resistance at high stress areas such as corners and molded edges. In addition, conventionally formulated unsaturated polyester compositions generally exhibit poor resistance to strong aqueous caustic and acid solutions and can readily degrade in polar organic media.
A method that has been applied for reducing the polymerization shrinkage, and for modifying the final physical properties of the crosslinked compositions, involves the addition of various high molecular weight thermoplastics such as homopolymers of various acrylates, methacrylates, styrene, and vinyl acetate; copolymers of styrene; copolymers of vinyl chloride and vinyl acetate; cellulose esters; saturated polyesters and polycaprolactone. These materials, however, increase the viscosity of the unsaturated polyester resin to such an extent that formulation of other components such as fillers and fiberglass with the resin becomes difficult or impossible. The high viscosity results in poor wet-out of fiber reinforcement and filler, and can impart undesirable physical properties to the final molded product.
Micropulverized polyethylene powder has also been reported as a shrink-control agent in unsaturated polyester molding compounds. The conventional procedure involves mechanically blending polyethylene powder with the polyester-monomer resin syrup along with fillers and fiberglass. A major problem associated with combining polyethylene powder with the resin in this fashion is the extreme difficulty in properly dispersing the powder in the resin phase. Poor dispersion causes porosity and craters which subsequently cause problems of poor physical properties and problems on painting.
U.S. Pat. No. 3,207,816 discloses that modification of reactive, essentially linear ethylenically unsaturated polyester resins, prior to cross-linking, by the addition thereto of an interpolymer of an .alpha.,.beta.-ethylenically unsaturated dicarboxylic acid or anhydride with a monoolefin having a terminal methylene group, results in linear alkyd resins having higher molecular weights than are readily obtainable by conventional procedures. The resulting linear alkyd, due to its improved viscosity, is said to impart many outstanding advantages to polyester resinous compositions prepared therefrom. This patent further discloses, however, that the amount of interpolymer or mixture of interpolymers employed may be from about 0.05% to about 10%, preferably about 0.2% to about 3%, by weight based on the total weight of resin forming components.